Electrolytic capacitors with increased available capacitance



Jan. 27, 1959 H. H. HILTON ET AL ELECTROLYTIC CAPACITORS WITH INCREASED AVAILABLE CAPACITANCE Filed Dec. 15, 1954 5 V/ L m C 0 Il M ,M w. wx N. .H... 0M FM 0 EN am ,M am f ww.. mm wf wm ,w m mf M M 0f M, 0 f l0 MW f 5 F C IMM H. C6 0 ,M WM om N5 WM HM I0 Nr. 0 40/ mm M mm ww 0C 2 nn f W C 0 W M w w w M M f'c/g. 2.

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ELECTROLYTIC CAPACI'lRS WITH INCREASED AVAILABLE CAPACHANCE Harry l-l. Hilton, Glens Falls, N. Y., and riltretl F. Torrisi, Chicago, Ill., assignors to General Electric Company, a corporation of New York v Application December 1S, 1954, Serial N o. 475,424

4 Claims. (Cl. 317-230) The present invention relates to electrolytic devices, and nmore particularly to electrolytic capacitors.

Electrolytic capacitors conventionally include an electrolyte which is in contact with a pair of electrodes, one

:as a whole.

lt is an object of the present invention to provide an electrolytic capacitor having electrodes, especially a cathode, which avoid the above disadvantages of prior vcapacitor electrodes, and which ar'iords increased capacitance in the capacitor unit.

It is a further object of the invention to provide la method of treating the electrodes and especially the ycathode of electrolytic capacitors to increase the available `capacitance produced in the capacitor unit.

A particularly notable advantage of electrolytic capacitors, as compared to the non-electrolytic type, resides in `the great capacitance per unit of the electrode area which it exhibits, especially at very low voltages. However, it has been found that the available capacitance in an electrolytic capacitor is not fully utilized, as indicated above, due mainly to the characteristics of the cathode of the capacitor unit. The capacitance or" the electrolytic capacitor can be considered, in eiect, as the resultant of the capacitances of the anode and the cathode :arranged in series. This is based on the fact that actually 'the electrolyte in an electrolytic capacitor is itself a .cathode adjoining the dielectric film on the anode surface, vwhile the usual metal electrode conventionally designated the cathode constitutes a second capacitor due to the dielectric lilm which usually forms on its surrace adjoining the electrolyte. Consequently, the capacitance of an electrolytic capacitor is the reciprocal of the sum of the reciprocals of its anode and cathode capacitances which can be expressed by the well known equation:

CMQ Ca-lcc where C, is the capacitance of the anode, CC is the capacitance of the cathode, and C is the capacitance of the unit. lt will be apparent from the above equation that the capacitance of the cathode is an important fact-or in achieving the total capacitance of which the unit is capable, since in no case can the capacitance or" a unit exceed that of its cathode.

it has been found, in accordance with the invention, particularly where silver is used as the cathode in an electrolytic capacitor, that unexpectedly and greatly improved results in increasing the available capacitance of the capacitor unit can be achieved by a novel treatment of the silver cathode, which may, for example, be in the form of a container or case enclosing the capac- #-2 itor unit. The vastly improved results in increased capacitance were accomplished by treating the silver cases in a hot concentrated sulfuric acid bath for a pro-longed period, and until the cases had a darkened, dull finish as compared to their bright glossy appearance prior to the severe acid treatment.

ln capacitors of miniature and microminiature scale, the extremely small effective surface area presented by the electrodes makes it even more desirable to utilize the maximum available capacitance of the capacitor unit, and the present method of increasing the cathode capacitance is therefore especially valuable in these small types of capacitors.

The invention will be best understood when talien in conjunction with the following description and the accompanying drawings, in which:

Fig. l is a cross-sectional View of an electrolytic capacitor embodying the present invention; and

Fig. 2 is a graph illustrating the improvement provided by the present invention as compared to the prior are devices.

As shown in Fig. l, the electrolytic capacitor may cornprise a substantially cylindrical container or case l made of silver which serves as the capacitor cathode. Cathode case l contains therein an electrolyte Z of known composition which may be present either with or Without an absorbent material (not shown) for retaining the electrolyte. The anode 3 arranged in the cathode case l is in contact with the electrolyte 2, the anode being formed of a metal not susceptible to corrosion by the electrolyte used in the capacitor, and being in the shape of a cylinder (as shown), a spiral, a coil, or other configuration. rThe anode 3 is provided with a dielectric film which may be formed by any well known method. Preferably, the anode is etched, in accordance with known processes, to produce a greater effective area in contact with the electrolyte and correspondingly increase the capacitance of the anode. The anode and cathode are provided with terminal leads d, d', in the usual manner, and the electrolyte 2 is sealed in case l by a suitable plug 5 of known construction.

The capacitor units for which the present silver cathodes are especially suited are those incorporating electrolytes which tend to be corrosive of the electrodes, unless the electrodes are made of a material inert or otherwise resistant to the action of the electrolyte. Examples of such electrolytes are those containing lithium chloride, and electrolytes composed of a mixture of ethylene glycol, distilled water and ammonium succinate or berate. Anode metals which resist corrosion of such electroyltes are therefore preferred for use in capacitors embodying the present invention, and among the types of materials which serve satisfactorily as anodes are film forming metals such as tantalum, niobium, alloys of tantalum and niobium, zirconium and titanium. Aluminum anodes (or cathodes) would not be suitable in such electrolytes since they are much less refractory than the above mentioned metals and would therefore be readily corroded by the electrolytes.

ln accordance with the invention, the silver cathode casing 1 is subjected to a severe acid pre-treatment before assembly oi the capacity unit in order to produce, at least on the inner face thereof, a dark and dull surface, and as a result of this treatment exceptionally marked increases in the total capacitance of the capacitor unit have been observed.

The acid used in the present process is concentrated sulfuric acid which may have a specitlc gravity as high as 1.84, and the temperature at which the acid treatment is carried out may vary between 50 C. and the boiling point of the sulfuric acid used. The boiling point of the acid thus may vary from about 290 C. in the case of sulfuric acid of 1.78 specic gravity, to about 330 C. for sulfuric acid of 1.84 specic gravity. The time of treatment of the silver electrode by the hot acid which is required to produce a dull, dark surface thereon may,`

actual capacitance of the samples is calculated from the equation C A=C 8V 1 -l-D2 where CA equals the actual capacitance, Ce equals the therefore, vary depending on the concentration of the 1 acid, the temperature of the acid, and, of course, ori the equwaent capacltance (1' e" the capacltau as I neasuied condition of the electrode material as received. by the z angle meten and equals tile dlsslpanon In a typical process carried out in accordance with factor' s m the actual Capacltance .CA 1S.the Sum .of the invention, silver cases as received from the supplier v the capacltans of the total Surfae .mcludmg- Opposl were placed in a vessel and covered therein with cold i0 Sides Ofthe Sampls the actual mslde capacltance of Concemated sulfuric acid (abo'ut 93% H0501.)l A the individual cases is calculated as one half of the actual vacuum was applied to the vessel to exhaust thel air capactacqouamed' trapped in the cases. The vessel was then placed over The dlsslpauon factor memloned above 1s the rail() a source of lieat and the temperature in the vessel was of the resistive component of the cathode current to me raised to about 130 C., which was maintained for aoapacltwe Cmpoqent of the cathode clirient Thus the Proximaly minutes- At the end of this period it increased dissipation factor characterizing the treated was observed that the silver cases became dark and dull Cathdes Shows that the reslstlve component (if the cath' in appearanc The hot ac1-d` Was then wanted? and ode impedance has been decreased by .the acid cleaning the thus treated cases were rinsed several times in water In other. Words the and treatment provldes for Improved mlm they were fee of acid- Thereafter the cass were conductivity between the electrolyte and the metal cathoven-dried at about 85 C. Ode m the capacltor Fig. 2 of the drawing graphically shows the improved The reasqns for the-une7pected1y great Improvement results provided by the present method of treating the in the capacitance obtained in the silver cathodes treated capacitor cathode. In the graph, the unit capacitance m accordance Wlth tile mventlon alie not fully under' in niicrofarads is plotted against the anode capacitance Stood' In part the ntcreased (fapacltan 1S no doubt in microfai'ads. Since it is clear from the capacitance due to removal of forelgn matenals such as grease from equation given above that the unit capacitance can never th. Surface of the Cathode and-to etching of the metal exceed that 0f the cathode Curve IV as drawn reim? which affords increased effective Contact between the sents a cathode of infinite capacitance. It is evident from i Cathode and the capacltor electrolyte' However these the graph that the closer tlie capacitance curve of a eects alom? do not appear to account for th? excellent cathode approaches curve IV, the greater is the amount results Obtained by the presept processm Vlew of the of available capacitance which can be produced in the fact that mechamcal roughenmg or eichmg of the caih' capacitor unit ode surface has not been found to give the marked in- Curves I and Il represent the measured capacitance of crease m cathode. capacitan@ aorqed by the present two groups of prior art silver cathode cases which have process It lsbhevqd thin the Ramon of the hot con' not been acid treated 01. have been subjected only ,[0 a centrated sulfuric acid with the silver cathode, besides comparatively short period of acid treatment which leaves having the clearnmg and neghglble etching eEeC-ts men' the cases with a bright surface. Curve III represents honed. above 10mm .ou th? Surface of .the sliver an the capacitance of capacitor units with silver cathode E Xceptlonauy stable dlelecmc mm of deslrabie propercases of the same type as those of curve II which have 40 ues' The lm. formed appears to be? a relitwely .gqod however, been subjected to a severe acid 'treatment in cnductor which 1S .pamularly desirable m .Ol-taining acordance With the invention. The area between curves hlgher capacitance Sme m the ilse of capcltorswlth I and II represents the possible variation in capacitance direct current the deslred capacltor eect 1S achleved obtainable with silver cathode cases known to the art solely by means of the Oxlde or other dlelectric mm at the present time. The material represented by curve forme@ on the anode and the Current Should pass with' H is more the exception` rather than* the wie in terms out hindrance between the electrolyte and the cathode. of capacitance produced by prior art cathodes. The nature of the min (.m the cathode formed by reaction From the graph it will be geen that curve 'HI repre with the sulfuric acid is not delinitely ascertainable due sentative of a cathode treated in accordance wgith the to lts extreme thmness but 1t may conceivably be a mm present invention, is substantially closer than the prior of Ag2SdAg2O or Agzsai or a combmatlon of these art cathode to the ideal capacitance illustrated b ruwe compoun s' IV and demonstrates the considerable increase inycanac- Another ativan-tage of the present-process 1--esldes in itance which is achieved bv the presentrprocess of tieatthe Improved mamtenane o-f the attained capacltan of ing electron/tic Capacitor Cthos' 1- the silver cathode. While it has been believed hereto- In further tests, three grou-Psio silver cases as received fre that only a clean bln-ight sllver catho-dek surface would from the supplier were given comparative vtests to de' gfllve thetlptlmulm capaclt-an of the umt experience has of the am is; si; naar uur, rite ing the capacitance values of cathodes one-lialf of each d nwar .l u Y rift in capacitance with the passage of time This degroup bein iven a severe tlf described bogve While the tlhenlafmin trglflnenrtms crease in capacitancehas also been found to characterize was left untreated The ouown table h tha g 1P capacitors incorporating .the usual film-forming cathodes Observed. L g S OWS l'eSU S such as aluminum on which oxide iilnis are formed. Such lnis are typical of the dielectric or resistive films to Group A Group B Group C Not Treated Not T e t d Treated Y Treated r ae Trleried Treated Capacitance Ce (mfd. f `g, tlOc l.; ,i Y Dissipation Factor, amggbaoi Saiilg.- 8.26? l 097g 855 6923? 476% Average Amai capacitance ci (mia.) t- 995 2,265 74.5 1,'560 ses i567 Average Actual Inside Capacitance (mfd.)- 49.5 1 132 37.2 785 41.7 303.5

Due to the characteristics of the Z angle meter by which the above capacitance measurements were taken,

which low cathode capacitance is attiibilitable,v and,in ore the over, these types of lms are relatively unstable iutliait with the passage of time they cause a further decrease in the cathode capacitance.

The present process has been found to overcome the above disadvantages to a considerable degree, due presumably to the thin film which apparently constitutes, at least in part, the dull, dark surface of the treated silver cathode. Not only does this iilm initially contribute to a marked increase in the capacitance of the cathode, but due to its stability serves to maintain indenitely the capacitance thus attained.

While certain embodiments of the present invention have been described, it is to be understood that various modifications may be made Without going beyond the scope of the invention. For example, the silver cathodes may be in forms other than that of containers; for exam* ple, they could be at or wound electrodes or any other shape. Silver electrodes herein disclosed may also be used as anodes at suiiciently low voltages, below about 0.1 volt. Further, the cathode containers may be of other metals than silver but having silver deposited on the inner surfaces thereof in contact with the electrolyte. Also, electrolytes other than those mentioned could be incorporated in capacitors in which the present cathodes are used, and may be of liquid, semi-solid or other form. Therefore, the appended claims are intended to cover all such equivalent variations as come Within the true spirit and scope of the foregoing invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. An electrolytic capacitor comprising, in combination, an electrolyte and a pair of spaced electrodes coacting with said electrolyte, at least one of said electrodes being formed of silverand being subjected to a pre-treatment with concentrated sulfuric acid at a temperature of between 50 C. and the boiling point of the acid for a suicient period to produce a dark, dull surface thereon, whereby the capacitance of said silver electrode and thereby the available capacitance of the capactor are increased.

Z. An electrolytic capacitor comprising, in combination, a silver cathode casing, an electrolyte contained in said casing, and an anode arranged in said casing and having a dielectric film formed thereon contiguous to said electrolyte, said silver cathode casing being subjected to a pre-treatment with concentrated sulfuric acid at a temperature of between 59 C. and the boiling point of the acid for a sucient period to produce a dark, dull surface thereon, whereby the capacitance of the cathode and thereby the available capacitance of the capacitor are increased.

3. An electrolytic capacitor comprising, in combination, an electrolyte, an anode formed of a metal selected from the group consisting of tantalum, niobium, zirconium, titanium and alloys or combinations thereof, said anode having a dielectric lm formed on the surface thereof, and a silver cathode having contiguous to said electrolyte a dull, dark surface produced by treatment of said silver cathode with hot concentrated sulfuric acid, whereby the capacitance of the cathode and thereby the available capacitance of the capacitor are increased.

4. A low voltage capacitor comprising, in combination, an electrolyte and ja pair of spaced electrodes comprising an anode and a cathode co-acting with said electrolyte, at least said anode being formed of silver and being subjected to a pre-treatment with concentrated sulfuric acid at a temperature of between C. and the boiling point of the acid for a sufficient period to produce a dark, dull surface thereon, whereby the capacitance of said silver anode and thereby the available capacitance of the capacitor are increased.

References Cited in the tile of this patent UNITED STATES PATENTS 2,616,953 Booe Nov. 4, 1952 2,617,863 Stinson Nov. ll, 1952 2,710,369 Booe June 7, 1955 OTHER REFERENCES Electrical Manufacturing, volume 46, Issue 6, pages 82 to 85, published December 1950.

Mellor: Treatise on Inorganic Theoretical Chemistry, volume 3, page 350, published by Longmans Green & Co., New York City, 1923. 

1. AN ELECTROLYTIC CAPACITOR COMPRISING IN COMBINATION, AN ELECTROLYTE AND A PAIR OF SPACED ELECTRODES COACTING WITH SAID ELECTROLYTE, AT LEAST ONE OF SAID ELECTRODES BEING FORMED OF SILVER AND BEING SUBJECTED TO A PRE-TREATMENT WITH CONCENTRATED SULFURIC ACID AT A TEMPERATURE OF BETWEEN 50* C. AND THE BOILING POINT OF THE ACID FOR A SUFFICIENT PERIOD TO PRODUCE A DARK, DULL SURFACE THEREON, WHEREBY THE CAPACITANCE OF SAID SILVER ELECTRODE AND THEREBY THE AVAILABLE CAPCITANCE OF THE CAPACITOR ARE INCREASED. 